THE VISUAL PIGMENTS 



at 538 m/^, resulting from this second exposure are shown by curve 2 

 in Fig. 6.7. Finally, to complete the bleaching, the cell was exposed 

 to *white* light (a 15W lamp) for 20 min. This caused further 

 density changes, maximal at 512 m/u (curve 3 in Fig. 6.7). 



400 



450 



500 550 



Wavelength in rr\\L 



600 



650 



Fig. 6.6. Comparison between the density spectra of pike and bleak 

 extracts, before and after total bleaching by white light. The vertical 

 dashed lines facilitate comparison of the two difference spectra. O, 

 measurements made consecutively from 380 to 680 m/^; •, return 

 measurements from 670 to 390 m/z. Pike extract, pH8-24; bleak 

 extract, pH 8-15; r=20°C. 

 (Dartnall, 1955) 



The total density changes caused by the orange-red light (two 

 exposures) and the white light are given by the algebraic sum of the 

 separate density changes, that is by the sum of the curves 1-3 in 

 Fig. 6.7. This sum is represented by the dotted curve. For compari- 

 son, the density changes which had been observed on the previous 

 day when an earlier sample of the same extract bleached to com- 



172 



